1. Field of the Invention
The present invention relates to a comparator circuit which can be used in various control apparatus or the like.
2. Description of the Prior Art
In various control systems, such as temperature control devices or the like control apparatus, it is a rare case wherein only a single input signal, such as a sensor output signal, status signal or the like is provided as input to the control apparatus. In most cases, a plurality of such signals are inputted to the control apparatus. In this connection, a comparator circuit is very often used as the interface circuit between the input signal source, such as one or more sensors, and the control apparatus which usually includes one more logical circuits.
Such a comparator circuit is usually implemented in a differential circuit arrangement, as is shown in FIG. 1. More particularly, transistors 10 and 20 are connected in a differential configuration having input terminals 100 and 200, respectively, connected to the bases thereof, while the emitters of the transistors 10 and 20 are connected in common to a current source 1. A diode 2 and a transistor 3, both serving as load elements, are connected in the collector circuits of the transistors 10 and 20. Numeral 4 denotes an output amplifier and symbol +V denotes a potential source. With such an arrangement, the output signal of the comparator circuit available from the amplifier 4 will become logic "1" or "0" in dependence upon the relationship in magnitude between the input voltages applied to the input terminals 100 and 200. When the potential level at the input terminal 100 is higher than that of the input terminal 200, collector current of the transistor 10 will be increased. Consequently, the collector current of the transistor 20 must be decreased and will flow to the amplifier 4, resulting in the logic "1" at the output of the amplifier 4. On the contrary, when the potential at the input terminal 100 is lower than that of the input terminal 200, the comparator circuit operates in the reverse manner to output logic "0" through the amplifier 4.
FIGS. 2a and 2b show typical arrangements of comparators which are employed when characteristics between the input and the output signals, such as illustrated in FIGS. 3a to 3c, are to be attained. In the case of the comparator arrangement shown in FIG. 2a, a signal from a signal source 101 is applied in common to the input terminals 100-1 and 100-2 of two comparators 5-1 and 5-2, while the other input terminals 200-1 and 200-2 of the comparators 5-1 and 5-2 are supplied with signals of predetermined amplitudes from signal sources 201 and 202.
With the input voltage from the signal source 101 represented by e.sub.S and the signal voltages from the signal sources 201 and 202 represented by e.sub.U and e.sub.L, respectively, it is assumed that e.sub.U &gt;e.sub.L. Further, it is assumed that each of the comparators has such characteristics that logic "1" is produced at the output when the upper input terminal 100-1 or 100-2 reaches a higher potential than that of the lower input terminal 200-1 or 200-2 and logic "0" is produced when the input conditons are inverted.
If the input signal e.sub.S is lower than e.sub.L or e.sub.S &lt;e.sub.L, then the signal e.sub.S will become lower than e.sub.U, namely e.sub.S &lt;e.sub.U from the assumption that e.sub.L &lt;e.sub.U. The outputs from both of the comparators 5-1 and 5-2 are both logic "0's." On the other hand, if the input signal e.sub.S is in the range of the signal levels between e.sub.L and e.sub.U, i.e. e.sub.L &gt;e.sub.S &lt;e.sub.U, the output from the comparator 5-1 is logic "1," while the output from the comparator 5-2 is logic "0." Under the condition that e.sub.U &lt;e.sub.S, then e.sub.L &lt;e.sub.S from the assumption that e.sub.U &gt;e.sub.L. This means that both outputs from the comparators 5-1 and 5-2 will become logic "1's." Thus, it will be understood that, when a sensor is employed as the signal source 101 and the output from the sensor as represented by e.sub.S is applied to the input terminals 100-1 and 100-2, the characteristics of the control output signal, such as shown in FIG. 3a or 3b, can be easily obtained by merely providing a simple logic circuit 50.
These characteristics are often utilized in an apparatus for determining whether an input signal is in a predetermined range or not. In such case, two comparators have to be used, as will be apparent from the foregoing description.
The circuit shown in FIG. 2b is so arranged as to obtain a so-called hysteresis characteristic. Operation of this circuit is conducted in a substantially similar manner as described above. In brief, the output signal from the comparator 5-1 functions to set or reset a flip-flop of the logic circuit 50, while the output from the comparator 5-2 is used to reset or set the flip-flop of the logic circuit 50, whereby the characteristic, such as shown in FIG. 3c, can be obtained. Such operating characteristic is essential for on-off or switching control elements employed widely in various practical applications.
In the case where a single comparator has to deal with two or more signals, it will be contemplated that the input signal sources 101 and 102 are alternatively connected to the comparator 5 with the aid of a change-over switch 90, as is shown in FIG. 4. In such case, since the output signal 101 or 102 is often at low level, as in the case of a sensor output signal, it is required to use a change-over switch with a high precision in order to assure a desired reliability. In practice, the switch 90 shown symbolically in FIG. 4 is constituted by a transistor or the like circuit. In order to meet the high accuracy or reliability requirement described above, the practical switching circuit will become necessarily very expensive to cancel out advantages obtained by using the single comparator.
As will be appreciated from the above description, an increased number of comparators, as well as a circuit of enlarged scale, are necessarily accompanied by correspondingly increased expenditure. Thus, in the conventional control circuit which requires usually a plurality of sensors, an the increased number of comparators required will involve a serious problem in practice. Particularly, implementation of such a control circuit as an integrated circuit is subjected to severe restriction in respect of the circuit scale from the economical viewpoint, which has hitherto provided obstacles in practicality particularly in the control system including a large number of sensors.